THE SHELL-LIMESTONE OF DURHAM. 153 



see all the specimens side by side, when perhaps some of the 

 now apparent differences will melt away through easy gradations 

 of size and contour. 



These Cytlieres, having a more or less elongate, kidney-shaped 

 outline, have thin valves, like those of BaircUa^ with internal 

 marginal lamina, and more or less overlapping dorsal and ventral 

 edges; but with this distinction, the two valves are not such 

 unequal contributors to the hinge-line as they are in Bairdia. 

 In the latter, the dorsal edge of the right valve is pared down, 

 as it were, and wholly overlapped by the opposite valve ; whereas 

 in this other group {Gytherideis^ " Monog. Tert. Entom. Engl.," 

 p. 46), the right valve is not always overlapped, retains its thick- 

 ness (it is indeed sometimes thickened), and often presents slight 

 folds or knobs, sufficient, in some instances, to help in marking 

 out one or both ends of the hinge-line on the closed carapace. 

 (See, for instance, " Monog. Cret. Entom.," pi. 6, fig. 18a). 



At the same time we must remember that the hingement of 

 these Entomostraca is variable, and that in Cytliere proper^ and 

 its sub-genera, there are endless modifications, forming gradual 

 passages from a hingement almost as simple as the mere meet- 

 ing of the dorsal edges in Cypris^ to the strongly-toothed hinge- 

 work of the thickest Gl]tliereis. In accordance with this varia- 

 bility of hinge, Cytherideis Jonesiana is, I may remark, certainly 

 nearer to Bairdia in the structure of the hinge-line than many 

 others of the sub-genus. Nevertheless, there is a distinct 

 thickened hinge-line in the specimens before us ; it occupies the 

 middle-third of the dorsal border. (PI. XI., fig. 24c.) 



The inclination to develope toothed or notched terminations 

 to the hinge-line, the sub-reniform shape and more obtuse ex- 

 tremities, the less uniform lenticularity of the lateral contour, 

 and a different plan in the arrangement of the " lucid spots" (as 

 far as I have yet had opportunities of noticing), are the cha- 

 racters which separate this groxx^ {Cytherideis) from its fellow sub- 

 genus Bairdia. These characters are not to be always readily 

 distinguished ; but when some are present, Ave may hazard the sup- 

 position that the others would appear in well-preserved specimens. 



VOL. IV. PT. II. TT 



